1. Field of the Invention
The present invention relates to a single crystal of lithium niobate or tantalate for optical use and its production method, an optical element using the single crystal and its production method. Particularly, it relates to a production method for stable growing a single crystal of its stoichiometric composition, which has excellent physical properties and which is used for an element utilizing polarization inversion, from a melt having a high Li concentration.
The present invention further relates to a process and an apparatus for producing an oxide single crystal by using a noble metal crucible of a double crucible structure. Particularly, it relates to a process and an apparatus for producing an oxide single crystal for stably growing a high quality and longitudinal crystal by rotation pulling.
2. Discussion of Background
Functional optical single crystals of which optical characteristics can be controlled by an information signal from outside such as electricity, light of stress, are now essential materials in various optoelectronics fields including optical communication, recording, measurement and optical-optical control. Particularly with respect to a certain single crystal of an oxide, the interaction between the optical characteristics and external factors is significant, and it is thereby used as a frequency-conversion element utilizing the nonlinear optical effect, or as an optical element utilizing the electrooptical effect, such as an electrooptical light modulator, a switch or a reflector.
Such a crystal is used as an element as it is originally grown, in many cases. However, with respect to some ferroelectric crystals, the directions of the dielectric polarization can be inverted by applying voltage thereto without destroying the crystals, and accordingly, their functions have been increased by inverting the polarization periodically.
For example, with respect to a frequency-conversion element, the wavelength can be converted by means of quasi-phase-matching (QPM) by periodically inverting the domain structure of the ferroelectric polarization. This method is effective from the viewpoint that the conversion can be carried out with a high efficiency at a wide wavelength range, and it is thereby expected as a frequency-conversion element to realize a laser light source having a wide range of wavelength covering from the ultraviolet and visible light region to the infrared light region, which is strongly desired in fields including optical communication, recording, measurement and medical care.
Further, with respect to an electrooptical element, according to a known literature (M. Yamada et al., Appl. Phys. Lett., 69, page 3659, 1996), an attention has been drawn to a cylindrical lens, a beam scanner and a switch, and an optical element forming a polarization inversion structure of a lens or a prism in a ferroelectric crystal, and polarizing laser light transmitted therethrough by utilizing the electrooptical effect, as new optical elements.
A single crystal of LiNbO3 or LiTaO3 (hereinafter referred to simply as LN single crystal or LT single crystal, respectively) is a ferroelectric which is used mainly as a substrate for a surface elastic wave element or for an electrooptical light modulator. It is transparent at a wide wavelength range of from the visible region to the infrared region, it can form a periodic polarization structure by applying voltage, it has optical nonlinearity and electrooptical characteristics which are practical to some extent, and further, a single crystal having a large diameter and a high composition homogeneity can be provided at a relatively low cost. Accordingly, an attention has been drawn to the LN single crystal or the LT single crystal also as a substrate for a frequency-conversion element by the above-mentioned QPM (hereinafter referred to simply as QPM element) or for an electrooptical element.
Heretofore, the LN single crystal available has been limited to one of the congruent melting composition with a molar fraction of Li2O/(Nb2O5+Li2O) of 0.485, containing nonstoichiometric defects of a level of several percent, including the substrate for a surface acoustic wave element, since the phase diagram of the LN single crystal has been known for a long time, and it has been conventionally considered that to produce a LN single crystal having a high composition homogeneity, it is preferred to grow the single crystal by rotation pulling from a melt with a molar fraction of Li2O/(Nb2O5+Li2O) of 0.485, which is of the congruent melting composition wherein the crystal and the melt are coexist in equilibrium state with the same composition. Further, as shown in a known literature (D. A. Bryan et al. Appl. Phys. Lett. 44, page 847, 1984), Mg is added in an amount of at least 4.5 mol % to the LN crystal of the congruent melting composition, with a purpose of increasing optical damage resistance. For the LT single crystal available has been limited to one of the congruent melting composition with a molar fraction of Li2O/(Ta2O5+Li2O) of 0.483, containing nonstoichiometric defects of a level of several percent, including the substrate for a surface acoustic wave element, since it has been conventionally considered that to produce a LT single crystal having a high composition homogeneity, it is preferred to grow the single crystal by rotation pulling from a melt with a molar fraction of Li2O/ (Ta2O5+Li2O) of 0.483, which is of the congruent melting composition wherein the crystal and the melt are coexist in equilibrium state with the same composition. Further, as shown in a known literature (F. Nitanda et al. Jpn. J. Appl. Phys. 34, page 1546, 1995), Mg is added in an amount of a level of several mol % to the LT crystal of the congruent melting composition, with a purpose of increasing optical damage resistance or shortening the cut-off wavelength. However, it has been known that the LT single crystal has a relatively high optical damage resistance even without Mg addition, as compared with the LN single crystal, and an adequate effect of improving the optical damage resistance by Mg addition is not always obtained.
In order to realize the QPM element, it is important to prepare a small element having a high efficiency. Downsizing and obtaining high efficiency of the element are significantly dependent on the characteristics of the material to be used, i.e. the material characteristics which the crystal essentially has, although they are greatly dependent on also the structure of the element. For example, the conversion efficiency of the QPM element is in proportion to the square of the nonlinear optical constant and the interaction length, and is in proportion to the fundamental wave power density. The interaction length and the fundamental wave power density are determined by the element design or accuracy of the preparation process, and may be increased by e.g. improvement of techniques. On the other hand, the nonlinear optical constant is a material characteristic which the material essentially has. Since LN is one of the most popular nonlinear optical materials, a large number of measurements of the nonlinear optical constant have been carried out for a long time. Of the LN crystals of the congruent melting composition which have been reported so far, the nonlinear optical constant d33 has been said to be usually from about 27 to about 34 pm/V at a wavelength of 1.064 xcexcm. However, the difference among reported values is surprisingly large, and it is twice between the highest and the smallest. These values are obtained by a relative measurement for obtaining the ratio in the nonlinear optical constant with a reference substance. However, the absolute value of the reference substance itself is not determined, and researchers use different values, and the difference is thereby so large. In the conventional measurement methods, the absolute value of the reference substance is based on the value obtained by absolute measurement for directly measuring the absolute value of the nonlinear optical constant. However, there is a significant difference in the obtained value between second harmonic generation (SHG) method and parametric fluorescence (PF) method which are representative absolute measurement methods. For example, d11 of quartz is 0.3 pm/V according to the absolute value scale based on SHG method, whereas it is 0.5 pm/V based on PF method, at a fundamental wavelength of 1.064 xcexcm. The absolute value of the nonlinear optical constant has been inadequate, and according to a known literature (I. Shoji et al., J. Opt. Soc. Am. B, 14, page 2268, 1997) for example, it has been known by careful absolute measurements by both SHG method and PF method, that the values by PF method which have been reported so far, were overestimated since e.g. influences of stray light at the time of measurement can not be completely excluded, and the same value can be obtainable essentially by either method. Recently, the absolute value has, at long last, become to be measured with a high accuracy, and with respect to the LN crystal of the congruent melting composition, including one having Mg added thereto, the nonlinear optical constant d33 has been corrected and reported to be from 24.9 to 25.2 pm/V. Further, with respect to the LT crystal of the congruent melting composition, d33 based on fundamental wave having a wavelength of 1.064 xcexcm, has been reported to be 13.8 pm/V according to the known literature (I. Shoji et al., J. Opt. Soc. Am. B, 14, page 2268, 1997).
Further, in the case of using the LN single crystal or LT single crystal for an electrooptical element, a high electrooptical constant is desired. The LN or LT single crystal has begun to be used as a material of the substrate for an optical element utilizing various electrooptical effects, since a single crystal having a high quality and a large diameter can be stably produced at a low cost, although the electrooptical constant of the single crystal itself is not particularly high among ferroelectric single crystals. The electrooptical constant of the LN or LT single crystal has been measured usually by means of Mach-zender interferrometer. With respect to the LN single crystal of the congruent melting composition which has conventionally been used, the electrooptical constants r13 and r33 have been reported to be about 8.0 pm/V and about 32.2 pm/V, respectively. With respect to the LT single crystal of the congruent melting composition which has conventionally been used, the electrooptical constants r13 and r33 have been reported to be about 8.0 pm/V and about 32.2 pm/V, respectively. Accordingly, the structure of the element using the single crystal having a high electrooptical constant r33, has a significant merit for downsizing and obtaining high efficiency of the element.
In recent years, studies to reduce the nonstoichiometric defects in the LN or LT single crystal of the congruent melting composition, i.e. studies to make the crystal composition ratio to be in the vicinity of the stoichiometric, has clarified that the nonstoichiometric defects decrease the nonlinear optical constant that the LN crystal essentially has, and besides, increase the applied voltage necessary for preparing a periodic polarization structure. For example, according to known literature (V. Gopalan et al. Appl. Phys. Lett. 72, page 1981, 1998), the polarization inversion voltage can be decreased to be at most 5 kV/mm by making the crystal composition to be in the vicinity of the stoichiometric composition. Likewise, the studies have been clarified that the nonstoichiometric defects increase the optical characteristics that the LT crystal essentially has, and applied voltage required for preparing a periodic polarization structure. For example, according to JP-A-11-35393, the photorefractive characteristic and the transmittance characteristic of light can be improved by making the crystal composition to be in the vicinity of the stoichiometric composition. Further, another known literature (K. Kitamura et al. Appl. Phys. Lett. 73, page 3073, 1998) reports that the polarization inversion voltage can be decreased to be a level of from 1.5 to 1.7 kV/mm by making the crystal composition to be in the vicinity of the stoichiometric composition.
Further, in order to practically utilize the LN single crystal of the stoichiometric composition, studies with respect to its growing method have been extensively made. For example, according to known literature (G. I. Molovichiko et al. Appl. Phys. A, 56, page 103, 1993), the LN crystal having a small defect density and a composition in the vicinity of the stoichiometric composition, can be obtained by growing the crystal from a melt of the congruent melting composition or the stoichiometric composition, having at least 6 mol % of K2O added thereto. Further, in order to practically utilize the LT single crystal of the stoichiometric composition, studies with respect to its growing method have been extensively made. For example, JP-A-11-35393 proposes that the LT crystal having a small defect density and a composition in the vicinity of the stoichiometric composition, can be obtained by growing the crystal from a melt of the congruent melting composition or the stoichiometric composition, having at least 6 mol % of K2O added thereto.
From a phase diagram of Li2O and Nb2O5 as shown in FIG. 2, it is known that a crystal having a molar fraction of Li2O/(Nb2O5+Li2O) in the vicinity of 0.500 can be grown by making the molar fraction of Li2O/(Nb2O5+Li2O) of the melt for growing to be from 0.58 to 0.60. However, as shown in the phase diagram, the melt composition ratio is extremely close to the eutectic point, and in the case where a crystal of a composition in the vicinity of the stoichiometric composition is grown from a melt of a composition having a higher Li concentration over the stoichiometric composition, the excessive Li component remains in a crucible along with the deposition of the crystal, and the composition ratio of Li and Nb in the melt gradually changes, whereby the melt composition ratio achieves the eutectic point soon after the initiation of the growing. Accordingly, in a case of employing Czochralski method (hereinafter referred to simply as CZ method) which has conventionally been used as a means of industrial mass production of a LN crystal having a large diameter, the solidification ratio of a crystal of the composition in the vicinity of the stoichiometric composition is as low as a level of 10%. Likewise, according to known literature (K. Kitamura et al. Appl. Phys. Lett. 73, page 3073, 1998), a crystal having a molar fraction of Li2O/ (Ta2O5+Li2O) in the vicinity of 0.5 can be grown by making the molar fraction of Li2O/(Ta2O5+Li2O) of the melt for growing to be from 0.58 to 0.59. However, from a phase diagram as shown in known literature (S. Miyazawa et al. J. Crystal Growth 10, page 276, 1971), the melt composition ratio is extremely close to the eutectic point, and in a case where a crystal of a composition in the vicinity of the stoichiometric composition is grown from a melt of a composition having a higher Li concentration over the stoichiometric composition, the excessive Li component remains in a crucible along with the deposition of the crystal, and the composition ratio of Li and Ta in the melt gradually changes, whereby the melt composition ratio reaches the eutectic point soon after the initiation of the growing. Accordingly, in the case of using CZ method which have been conventionally used for industrial mass production of a LT crystal having a large diameter, the solidification ratio of the crystal is estimated to be as low as a level of 10%.
In order to raise this low solidification ratio, JP-A-10-274047 proposes a method of growing while continuously supplying starting material (hereinafter referred to simply as continuous supply method. Specifically, in the above method, the molar fraction of Li2O/(Nb2O5+Li2O) in the melt for growing is adjusted to be from 0.585 to 0.595, a crucible having a double structure is employed, a crystal is pulled up from an inner crucible, and the weight of the crystal being pulled up is measured at all times to obtain the growth rate, and a powder having the same component as the crystal is continuously supplied between an outer crucible and the inner crucible in the same amount as said rate. By employing this method, a longitudinal crystal may be grown, and the crystal solidification ratio of 100% based on the amount of supplied material will be realized. Likewise, in order to raise this low solidification ratio, a continuous supply method has been reported, for example, by a known literature (Y. Furukawa et al. J. Crystal Growth 197, page 889, 1999). Specifically, in the method, the molar fraction of Li2O/(Ta2O5+Li2O) in the melt for growing is adjusted to be from 58.0 to 59.0, a crucible having a double structure is employed, a crystal is pulled up from an inner crucible, and the weight of the crystal being pulled up is measured at all times to obtain the growth rate, and a powder having the same component as the crystal is continuously supplied between an outer crucible and the inner crucible in the same amount as said rate. By employing this method, a longitudinal crystal may be grown, and the crystal solidification ratio of 100% based on the amount of supplied material will be realized.
Further, the LN or LT single crystal is often used as a QPM element. As an important process technique to obtain a high efficiency, a technique of producing a periodic polarization inversion domain with a high accuracy may be mentioned. Namely, in order to obtain the maximum nonlinear optical characteristics, the ratio of the width of the polarization inversion (hereinafter referred to simply as polarization inversion width) is made to be 1:1. The polarization inversion width varies depending upon the phase matching wavelength of a frequency-conversion element to be obtained. For example, with respect to phase matching at a longer wavelength in e.g. the infrared region, the polarization inversion width is several tens xcexcm. The polarization inversion voltage of the LT single crystal of the congruent melting composition is said to be at least 21 kV/mm according to known literature (K. Kitamura et al. Appl. Phys. Lett. 73, page 3073, 1998). Further, the polarization inversion voltage of the LN single crystal of the congruent melting composition is said to be at least 21 kV/mm.
The LN or LT single crystal of the congruent melting composition is a crystal having a high non-linearity, among existing nonlinear optical crystals as shown in FIG. 3. However, its non-linearity is still inadequate in the case of practically preparing an element. Along with improvement of the level of performance of element design and improvement of accuracy in the preparation process in recent years, a significant improvement of the element characteristics will no longer be expected only by improving the process, and it has thereby been desired to make the constant d itself to be a higher value.
However, it has been gradually found that crystal growing method of pulling up the crystal from a melt having a higher Li concentration over the congruent melting composition by means of continuous supply method, has a significant problem in the yield from the industrial viewpoint. Namely, the present inventors have found that the composition of the growing crystal greatly depends on the composition ratio of the melt, in the case of using a melt having a high Li concentration, different from the case where a crystal is grown from a melt of the congruent melting composition. This means that it is necessary to grow a crystal from a melt having the same composition ratio always kept, in order to grow a crystal having uniform optical characteristics and good optical homogeneity with a high reproducibility. In the case of LN or LT crystal, the nonlinear optical constant, the voltage required for forming a periodic inversion structure, and the electrooptical constant, are sensitive to the crystal composition ratio, and accordingly, in order to obtain maximum properties, the molar fraction of Li2O/(Nb2O5+Li2O) or Li2O/(Ta2O5+Li2O) of the crystal to be extremely close to 0.500.
For example, the continuous supply method has such a characteristic that the composition can be excellently controlled from the initiation until the completion of the growing, however, the determination of the composition ratio of the melt at the initiation of the growing is very important, and if the initial setting is different from the desired composition of the melt, the nonlinear optical constant d33 or the inversion voltage required for the entire grown crystal is not satisfied. To prevent such, it is possible to correct the deviation by pulling up a small crystal before the growing, confirming the composition ratio of the melt from the composition ratio of the crystal, and adding insufficient component. However, it takes at least several days to grow the small crystal and confirm its composition ratio, whereby the productivity will significantly decrease. Further, although the continuous supply method is an extremely effective method for composition control, a slight amount of the material may evaporate from the surface of the melt kept to be at a high temperature, in the case where the growing time is so long as a level of from several days to a week. The change of the crystal composition with time due to the evaporation, can not be ignored in the case where it is required to grow a crystal of the stoichiometric composition wherein the composition is controlled to be completely homogeneous. It is extremely difficult to grow a crystal having the same characteristics with a high yield, due to this deviation of the crystal composition, and accordingly, the technique to grow a LN or LT single crystal of the completely stoichiometric composition and having no defect, from a melt having a high Li concentration, has not been put to practical use industrially.
Further, it is extremely difficult to completely form a polarization inversion width ratio of 1:1 with a high reproducibility, with the LN or LT crystal of the congruent melting composition. Namely, by voltage applying method, a periodic electrode is provided on one side of the LN or NT single crystal of the congruent melting composition of z-cut, and a uniform electrode is provided on the opposite side, and pulse voltage is applied through these electrodes, for polarization inversion at the part directly under the periodic electrode, toward the z-axis direction. However, the inversion polarization width and the electrode width are not always the same, and the preparation error is significant. Further, there are such problems that the polarization inversion widths are different between both sides of the z-cut crystal or the inversion may be broken during the formation of the polarization inversion toward the z-axis direction on the opposite side, whereby an ideal polarization inversion width range has not been obtained.
The polarization inversion width required for phase matching is about 3 xcexcm for a use for shorter wavelength ranging from the visible region to the ultraviolet region, and an element for shorter wavelength is more difficult to prepare as compared with an element for longer wavelength. However, even with respect to a QPM element for longer wavelength, which can be relatively easily produced, an ideal element has not been achieved yet. One reason is a high applied voltage required for the polarization inversion of the LN or LT single crystal of the congruent melting composition (hereinafter referred to simply as polarization inversion voltage). The polarization inversion voltage is so high as at least 21 kV/mm, and this high inversion voltage makes it difficult to form a complete polarization inversion in a case where the thickness of the substrate is at least 0.5 mm, and no polarization having a good accuracy, capable of realizing an element, has not been produced if the thickness is at least 1.0 mm, although a polarization inversion grating may be formed on the entire substrate when the thickness is less than 0.5 mm. Further, even if the thickness of the substrate is less than 0.5 mm, a polarization inversion period of several xcexcm, for shorter wavelength, has not been realized. Particularly in the case of a LN crystal of the congruent melting composition having at least 5 mol % of MgO added thereto, since the internal electric field is high, the hysteresis curve (P-E curve) of the ferroelectric has poor symmetry, and further, since the rising of the P-E curve is gentle and not steep near the anti-electric field, the inversion of spontaneous polarization may be poorly controlled when electric field in the direction opposite to the spontaneous polarization is applied thereto from the outside. Further, in the case of the LN crystal of the congruent melting composition having at least 5 mol % of MgO added thereto, the electric resistance will decrease by at least about 3 to 4 orders of magnitude, as compared with a case where no MgO is added, and accordingly, it will be difficult to subtly control the applying voltage, and it will be more difficult to make the polarization inversion width ratio to be 1:1. It is said that this problem may be overcome by employing corona discharge method for polarization inversion, however, the problem in the thickness of the sample for the polarization inversion has still been unsolved. Likewise, in the case of a LT crystal of the congruent melting composition having MgO added thereto, since the internal electric field is high, the P-E curve of the ferroelectric has poor symmetry, and further, since the rising of the P-E curve is gentle and not steep near the anti-electric field, the inversion of spontaneous polarization may be poorly controlled when electric field in the direction opposite to the spontaneous polarization is applied thereto from the outside. Further, in the case of the LT crystal of the congruent melting composition having MgO added thereto, the electric resistance will decrease as compared with a case where no MgO is added, and accordingly, it will be difficult to subtly control the applying voltage, and it will be more difficult to make the polarization inversion width ratio to be 1:1.
With respect to an electrooptical light modulator utilizing the electrooptical effect of a ferroelectric single crystal, an optical element having a polarization inversion structure of a lens or a prism formed on the LN or LT single crystal and polarizing laser light transmitted therethrough by utilizing the electrooptical effect, and a cylindrical lens, a beam scanner and a switch, it is important to prepare a small element having a high efficiency, to realize a new optical element. Also with respect to such an element utilizing the electrooptical effect, downsizing and obtaining high efficiency greatly depend on the characteristics of the material to be used, although they depend also on accuracy in preparation of the element structure. For example, performances of an optical element utilizing the electrooptical effect of the LN or LT single crystal having an inversion of the refractive index formed by the polarization inversion structure, are determined by the design of the polarization inversion structure of a lens or a prism, accuracy of the process for preparing the polarization inversion structure, and the electrooptical constant which the material has. With respect to the conventional LN or LT crystal of the congruent melting composition, it has been difficult to control the polarization inversion structure, since a high applied voltage is required for polarization inversion. Further, the electrooptical constant is a characteristic which the material essential has, and is considered to be difficult to improve in the same crystal. Further, the optical damage may be a big problem depending on the wavelength or the intensity of the light to be used, and in such a case, a crystal having at least 5 mol % of MgO added to the LN single crystal of the congruent melting composition, was expected to be excellent in optical damage resistance. However, such a crystal has a problem in material characteristics that the inversion of the spontaneous polarization is poorly controlled in the same manner as the preparation of the QPM element, whereby preparation of a polarization inversion structure of a lens or a prism having a good accuracy has not been realized. Further, although the LT single crystal of the congruent melting composition is believed to have a higher optical damage resistance than the LN single crystal, the optical damage may be a big problem depending upon the wavelength or the intensity of the light to be used. Even a crystal having at least 5 mol % of MgO added to the LT crystal of the congruent melting composition has insufficient optical damage resistance, and due to problems in material characteristics that the inversion of the spontaneous polarization will be poorly controlled in the same manner as the preparation of the QPM element, preparation of a polarization inversion structure of a lens or a prism having a good accuracy has not been realized.
The present inventors have conducted extensive studied to accomplish the above objects, and as a result, they have found the following. Namely, by adding to a melt at least one element selected from the group consisting of Mg, Zn, Sc and In, having substantially no absorption at the visible light region, in a total amount of from 0.1 to 3 mol % based on the total amount of the at least one element, Nb and Li, or the total amount of the at least one element, Ta and Li, a small polarization inversion voltage can be obtained without decreasing the nonlinear optical constant d33 and the electrooptical characteristic r33, the defects of Li can be compensated by said third element, and even with a single crystal of lithium niobate or tantalate having a certain level of nonstoichiometric defects, although having a composition in the vicinity of the stoichiometric composition, the same nonlinear optical constant, applied voltage required for preparing the periodic polarization structure, and electrooptical constant, as those of the perfect LN or LT single crystal having a molar fraction of Li2O/(Nb2O5+Li2O) or Li2O/(Ta2O5+Li2O) of 0.500, can be obtained; and further, this means is effective for a wide range of single crystals having a molar fraction of Li2O/(Nb2O5+Li2O) or Li2O/(Ta2O5+Li2O) of at least 0.490 and less than 0.500, and the present invention has been accomplished on the basis of these discoveries.
Czochralski method (CZ method) has been conventionally known as a method for growing a large single crystal of high quality. The CZ method is a method for producing a single crystal suitable for growing a large crystal of the congruent melting composition, wherein a seed crystal is contacted with a melt filled in a crucible, and the seed crystal is pulled upward while being rotated, for growing a single crystal below the seed crystal.
This method is now most commonly used industrially, for both oxide single crystal and semiconductor single crystal. However, when it is attempted to grow a longitudinal single crystal having a larger diameter at a low cost, the capacity of the crucible is limited, and accordingly, continuous pulling method of supplying a material into the crucible while pulling the single crystal up, has been devised, and various methods have been tried, including double crucible method.
This method is to produce a single crystal by employing such a structure that in the usual crucible, another crucible or cylinder having an opening for melt flow and having a small inner diameter, is arranged, wherein the outer crucible is for supplying a material, and the single crystal is pulled up from the inner crucible and grown (JP-A-57-183392, JP-A-47-10355).
For growing a semiconductor crystal such as Si or GaAs, a method of introducing a material in an amount corresponding to the degree of growth to the outer crucible when the single crystal being pulled up has grown to have a predetermined diameter, and its practical use is being considered. However, this is mainly for uniform addition of a dopant to obtain a longitudinal single crystal or to homogenize the material characteristics in the production of the semiconductor single crystal (JP-A-63-95195, Japanese Patent No. 2729243).
Also in the case of an oxide single crystal, a method for producing a crystal by means of a double crucible, similar to the production of a semiconductor single crystal, has been proposed. The method is mainly for growing a crystal of a composition different from the composition of the melt, which is difficult to grow by the CZ method on principle, and the method is expected to be excellent and being developed.
For example, a method is known for producing a crystal at a constant rate with the temperature and the composition of the material melt kept to be constant, by supplying material pellets between the outer crucible and the inner crucible, in order to overcome the low growth efficiency or variation of the crystal growth condition due to decrease in temperature and height of the melt required for the progress of the crystal growth, which have been problems in TSSG method which is one of solution pulling methods (JP-A-4-270191).
FIG. 4 is a schematic diagram illustrating a double crucible method of material supply type, which has been developed to make the pulling rate of a lifting and lowering head 7 by a crystal pulling shaft 6 and the falling rate of a melt 9 to be constant, by supplying material pellets 10 between an outer crucible 2 and an inner crucible 3 at a constant rate. In this method, several problems in the TSSG method have been overcome. In this method, a heater 4 is arranged at the outside of a double crucible 1, and the inner crucible 3 is provided with large holes 12, and a single crystal 11 is grown from a seed crystal 8 while dropping the material pellets 10 to the material melt 9 between the outer crucible 2 and the inner crucible 3 through a supply tube 5.
Further, a method for producing an oxide single crystal of high quality, by employing a noble metal crucible having a double structure, which functions also as a container generating heat by high frequency induction heating, to minimize the change of the temperature of the melt due to heating by the high frequency induction of the crucible, has been known, although the material is not supplied in this method, in order to overcome the problem of the change of the temperature during the growing of a crystal of the congruent melting composition wherein the compositions of the melt and the grown crystal are the same (JP-A-4-74790).
FIG. 5 is a schematic diagram illustrating the above method employing a double crucible made of a noble metal. A melt 15 of an oxide is put in an outer crucible 13, and in the outer crucible 13, a cylinder 14 having a smaller diameter than the inner diameter of the outer crucible 13 is arranged, however, this is to stabilize the temperature of the melt, and a means of supplying a material for growing a longitudinal crystal is not arranged. Further, in order to grow a crystal as long as possible, the shape of the outer crucible 13 is such that its height is substantially the same as or higher than its diameter.
Further, in order to grow a LiNbO3 single crystal of the stoichiometric composition having a molar fraction of Li2O/(Nb2O5+Li2O) of 0.50, which can not be grown by the conventional CZ method, a method for producing a single crystal employing a double crucible, wherein a melt of a composition having an excessive Li component with a molar fraction of Li2O/(Nb2O5+Li2O) of from 0.58 to 0.60 is preliminarily prepared in the inner crucible, a crystal of the stoichiometric composition is deposited therefrom, and an apparatus for continuously supplying a material powder prepared to have the same stoichiometric composition as the deposited crystal at the same time as the deposition of the crystal, is arranged, has been developed, and LiNbO3 single crystals having a homogeneous composition over the entire crystal, and having a composition in the vicinity of the stoichiometric composition, have been grown (K. Kitamura et al., Journal of Crystal Growth vol. 116, 1992, pages 327-332; Oyo Butsuri (Applied Physics) vol. 65, No. 9, 1996, pages 931-935).
FIGS. 6 and 7 are schematic diagram illustrating the above method. The change of the growth weight per unit time is measured by a load cell (52 in FIG. 6 or 27 in FIG. 7) for detecting the weight of the crystal, and a material powder in an amount corresponding to this change is supplied between an outer crucible (56, 19) and an inner crucible (55, 20) through a material supply tube (53, 22) arranged in such a manner that the angle to the vertical is from 65 to 76xc2x0. In either apparatus, the material is supplied automatically, and the supply amount of the material in a container for preserving the material is controlled by a piezo oscillator 54 in FIG. 6, or by a screw in FIG. 7.
Here, the diameter of the crystal to be grown is from about 1 to about 1.5 inches, and the shapes of the inner crucible and the outer crucible to be used for growing are such that the ratio of the diameter of the inner crucible to the diameter of the outer crucible is 0.5. Further, the crucible is not rotated in the apparatus shown in FIG. 6, and in the apparatus shown in FIG. 7, the crucible is extremely slowly rotated at a low rate of from about 0.1 to about 0.3 rpm in the direction opposite to the rotation of the crystal, for growing crystal, with a purpose of homogenizing the supplied material and the melt.
In the above method for producing an oxide single crystal which has conventionally been known, what is significantly different from the method for producing a semiconductor single crystal, is that a noble metal crucible which will not react with the melt is used for growing the crystal, and the weight of the material to be automatically supplied to the outer crucible has to be more precisely controlled since the amount of the crystal growth per unit time is small.
Accordingly, it is one of big problems to be overcome from industrial viewpoint to develop technique for growing a large and longitudinal crystal by using a crucible as small as possible, and to develop a growing technique by which the crucible can be used many times, since the noble metal crucible is likely to be deformed and it is extremely expensive.
As mentioned above, in the case where a crystal is grown without supplying the material, as shown in FIG. 5, there is a limit to making the crystal longitudinal. To grow a longitudinal crystal, it is necessary to prepare a crucible having a large diameter and to melt a large amount of the material. However, the crystal can not be made longer than the amount corresponding to the weight of the material preliminarily charged, and the noble metal crucible is extremely expensive, and accordingly the crystal will be rather costly, and the merit of the growing at a low cost by making the crystal longitudinal will be offset.
Further, in this case, the height of the surface of the melt decreases along with the progress of the growth, and thermal growth environment will gradually change, whereby the crystal growth interface may change, and accordingly, deterioration in quality is caused such as introduction of unfavorable crystal defects or distortion of the crystal. Such a problem can not be overcome even when a noble metal crucible with a double structure is employed, in the case where the material is not supplied in an amount corresponding to the weight of the grown crystal.
Accordingly, as shown in FIGS. 6 and 7, the double crucible method with material supply has been developed as a method to overcome the above problems, however, several problems has been found. For example, in this method, by employing a double crucible structure, the change in the temperature of the melt in the inner crucible can be made small, whereby defects such as growth striations observed in the obtained single crystal can be decreased, such being advantageous; on the other hand, the temperature gradient of the melt in the inner crucible in the diameter direction will be extremely gentle, and the shape of the crystal growth interface will be significantly different from one obtained by using a conventional single crucible, whereby it will be difficult to control the crystal growth interface and the crystal diameter which are important to grow a crystal of high quality.
FIG. 7 illustrates an example wherein the crystal growth interface is convex to the melt, and the crystal diameter can be well controlled. However, the growth interface is closely related with the relation between the size of the growing crystal and the size of the inner crucible, the thermal conductivity of the crystal, and the presence or absence of a dopant, and accordingly, in the case where the temperature gradient of the melt in the inner crucible in the diameter direction is extremely gentle, some device is required to control the shape of the crystal growth interface to be flat or convex to the melt. However, even if the conventional rotation of the crucible is carried out with a purpose of homogenizing the melt, no effect of forcibly controlling the growth interface can be obtained with an extremely slow rotation in the opposite direction to the rotation of the crystal at a low rate of from about 0.1 to about 0.3 ppm.
Further, with respect to the known methods for producing a single crystal by using a double crucible, as shown in FIGS. 4 and 5, the shape of the outer crucible is such that its diameter is substantially the same as the height of the crucible, and in the outer crucible, a crucible or a cylinder having a diameter and a height smaller than the outer crucible, having a hole, and called an inner crucible, is arranged.
It has been known that the proportion of the diameter of a crystal of high quality capable of being grown is usually about half relative to the diameter of the crucible. Accordingly, when a simple comparison is made with respect to the size of the crucible required for growing a crystal of the same size, the amount of a noble metal which is expensive, will be large in the case where a double crucible is used as compared with the case where only single crucible is used. Further, deformation of the noble metal crucible is significant after growing the crystal, according to the heating method or the shape of the double crucible, whereby the expensive noble metal crucible has to be repaired after every use of from several times to several tens times. Accordingly, if a double crucible having a larger size is used, the noble metal is far expensive as compared with the material, whereby the single crystal will be rather costly.
Further, there are several problems with respect to the methods of supplying the material in the method for producing a single crystal by using a double crucible, which have been conventionally reported. In the case where the material is supplied in the form of pellets as shown in FIG. 4, the weight of the pellets is heavy as compared with the weight of the powder. Accordingly, the supply of the material through the supply tube, i.e. falling of the material, is carried out relatively smoothly, and the material will not clog up the supply tube. However, the pellets will be supplied intermittently as compared with the continuous supply of the powder material, whereby the change in the temperature along with the material supply tends to be significant.
On the contrary, when the powder material is supplied by the method shown in FIG. 6 or 7, although there will be few problems in the intermittent temperature change along with the material supply, the material is likely to deposit on the supply tube during the supply, whereby the material tends to clog up the supply tube, depending on the size or the calcination condition of the powder. The supply tube is arranged in the growing furnace and is not transparent, it is thereby difficult to observe if the material clogs up the tube during the growing, and the clogging may not be noticed until the growing has completed.
Further, the crystal which is more difficult to grow from a melt by pulling, usually requires a lower growth rate and a smaller crystal diameter. In such a case where the amount of the crystal growth is small per unit time, it is necessary to supply a powder material having a correspondingly small particle size in a small amount. However, in such a case, the powder material may not fall into the crucible, but be flown up. Further, as shown in FIGS. 4, 5 and 6, in a case where the crucible is not rotated, if the material is always supplied to a certain specific portion between the outer crucible and the inner crucible, the crystal may precipitates from said portion, or the grown crystal may have non-uniformity in quality since no adequate melting and homogenization of the material may be carried out.
The method for producing a single crystal by using a double crucible, which has conventionally been used for growing an oxide single crystal, has some advantages to overcome the problems of the conventional Czochralski method, in principle. However, a means of supplying a material in an amount corresponding to the weight of the grown crystal, is not provided, or even if said means is provided, a method for industrially producing an oxide single crystal of high quality stably at a low cost, has not been achieved.
The present inventors have conducted extensive studies to achieve the above objects, and as a result, they have found the following. Namely, in the process for producing an oxide single crystal by rotation pulling by means of a double crucible made of a noble metal, by precisely controlling the method of arranging the material supply tube and the method of supplying the material, the method of preparing the material powder, the shape of the double crucible and the relation between the inner crucible and the outer crucible, rotation of the crucible and the like, it becomes possible to grow a crystal of high quality having a large diameter and being longitudinal, stably at a low cost, with respect to a crystal of the congruent melting composition or another nonstoichiometric composition, which has been considered to be difficult to grow, and the present invention has been accomplished on the basis of these discoveries.
Namely, the present invention provides a process for producing an oxide single crystal by rotation pulling by means of a double crucible made of a noble metal, consisting of an outer crucible made of a noble metal, and a cylindrical inner crucible for intersecting the surface of a melt in the outer crucible and connecting the melt at the bottom of the melt, which process comprises pulling a single crystal from the inner crucible while directly measuring the weight of the growing crystal for growing, simultaneously supplying a gas into a closed container, supplying a powder material preserved in the closed container between the outer crucible and the inner crucible through a supply tube in the same amount by weight as the crystal growth, and growing the crystal while rotating the double crucible.